Procedure for the realisation of ceramic manufactures, in particular, porcelain stoneware tiles and trim pieces, with anti pollution and anti-bacterial properties and products thereby obtained

ABSTRACT

This invention falls into the field of ceramic manufactures, in particular tiles and trim pieces made of porcelain stoneware. Titanium dioxide is applied to the glaze and to the layer covering the tiles so that a photocatalytic oxidization action can be produced acting against the polluting and bacterial agents present in the atmosphere.

TECHNICAL FIELD

The invention relates to a procedure for the realisation of ceramicmanufactures, in particular, porcelain stoneware, single-fired,monoporous or double-fired tiles and trims pieces with anti-pollutionproperties and products thereby obtained.

BACKGROUND ART

For some time now, it has been common practice to produce manufactureswith anti-bacterial and anti-pollution properties destined for thebuilding industry with a wide range of applications such as,construction conglomerates, panelling for road networks and self-lockingblocks for paving.

These manufactures are constituted, substantially, of a cement-basedconglomerate whose mass contains particles of titanium dioxide, TiO₂, aproperty of which is its capacity to reduce the polluting agents presentin the surrounding air.

Of these polluting agents, particular attention should be focussed onthe polycyclic aromatic hydrocarbons (PAH) derived from the incompletecombustion of organic materials, wood, coal, oil, and its derivatives,and also the nitrogen oxides (NOx) present in the exhaust fumes fromheating systems, industrial plants, motor vehicles, industrial wastesand pesticides. The abatement of the level of environmental NOx reducesboth the possibility of acid rain and the presence of nitrates which areharmful to humans and vegetation. The bacteria which can be attacked bythe presence of TiO₂ include, for example, Staphylococci and EscheriaColi. This result is due to the fact that the ultraviolet radiation ofsolar light, together with the humidity, interacts with the titaniumdioxide particles, leading to the production of active oxygen whicheffectively oxidises the aforesaid polluting and bacterial agentspresent in the atmosphere. The products of the aforesaid oxidisation areremoved by water, either rainwater or washing water, and also by thealkaline nature of the cement conglomerates which, until now, have beenrealised with photocatalytic properties. Moreover, the aforesaid removaland elimination of the polluting products prevent their stagnation onthe surface of the said manufactures, ensuring the original colours andtheir attractive appearance are maintained over time.

The drawback of the commonly-known manufactures with photocatalyticproperties lies in the cement base of the said products which cause thesaid manufactures to have a rough surface and, consequently, limitedpossibility of high quality aesthetic variants and, consequently,limited possibility of application in the field of external wallcoverings for the building industry.

Much higher aesthetic quality, for external wall coverings, is offeredby manufactures of a ceramic nature, in particular porcelain stonewaretiles or those made of other pastes, such as single-fired or monoporousmaterials; however, until now, as far as the applicant is aware, no suchmaterial has been realised with polluting and antibacterial agentabatement properties. The main reason for this lies in the fact that theproduction of ceramic tiles requires firing treatments involvingextremely high temperatures which, in the case of the porcelainstoneware manufactures, reach 1,200° C., unlike the production cycle ofcement-based manufactures, which are produced by mixing and subsequentcompaction, without requiring any thermal treatments.

It is known that when the TiO₂ in the form of Anatase reaches 900° C.,it is transformed entirely into Rutile, which, it has been demonstratedthrough experimentation, to be less effective than Anatase inparticularly critical pollution situations such as on roads with a greatdeal of traffic. Moreover, it has been observed that, in the productionphase, the transformation of Anatase into Rutile gives the glaze onceramic manufactures a yellowish colour.

The applicant's studies have concentrated on the way in which the TiO₂can be made to coexist with the aforesaid extremely high temperatureswithout the decay of its photocatalytic properties, in addition toensuring the photocatalytic effect of the ceramic tiles obtained in thisway do not cause a decline in the high aesthetic quality of the saidtiles.

The applicant's research and experiments have lead to differentconsiderations that are analysed hereunder.

The total presence of TiO₂ in a finished tile (as a percentage thereof)which provides the best results varies within a range of 1-25% of thetotal weight of the applications (glazes, silk-screening, engobe, etc.).It should be highlighted that the photocatalytic reaction of the TiO₂ tothe polluting and bacterial agents does not involve the consumption ofthe said TiO₂, which means the efficacy of its action remains constantover time. It should also be noted that TiO₂ can be mixed with both theglaze and the engobe (a covering formed of a thin layer of atomised clayapplied to the partially dried clay) and can also be applied with thesilk-screening pastes. The effectiveness of the oxidisation exerted bythe TiO₂ on the bacterial and polluting agents increases in the eventthat a photo-reflecting layer is applied, either beneath the layer ofglaze covering the tile or with the said covering layer; for example,both white pigments and silica particles mixed with the glaze can beused; in this way, the rate at which the solar light penetrates thetiles is increased and this increases the photocatalytic effect exertedby the TiO₂.

It is known that TiO₂ converts NOx into nitrate ions which, uponoxidisation, become Sodium and Calcium nitrates, which are not noxious,and which precipitate in the form of salts; the latter are removable bysimply washing with water. From this came the idea that this washingaction would be increased by an increase in the exposed surface,therefore the creation of micro channels on the surface of the tilewould facilitate the action of the water (rain water or washing water)when removing the products of the oxidisation caused by the pollutants.

In parallel, it was observed that the increase in the surface exposed tothe light increases the photocatalytic effect of TiO₂ and therefore itappeared evident to the applicant that it was also necessary to create,on the surface of the tiles, a plurality of non-uniform, micro unevenareas with the dual aim of permitting the solar light to hit the tilesfrom any direction and permit the air to better fix the NOx which isdecomposed by the ultraviolet radiation of the daylight.

To permit an efficacious retaining action of the gas developed duringthe night, while awaiting the daylight, the need to provide the tileswith materials able to store the said gas was considered. To this end,experimentation showed that the materials that absorb well are Zeoliteand Petalite minxed with the glaze, or Magalite added to the traditionalclays of which the tile base is composed.

DISCLOSURE OF INVENTION

The aim of this invention is to identify a procedure capable of enablingthe production of ceramic manufactures, in particular porcelainstoneware, single-fired, monoporous or double-fired tiles and trimpieces, with photocatalytic properties for the reduction of ambientpollution. In particular, the procedure for the realisation of ceramicmanufactures, in particular porcelain stoneware tiles and trim pieces,with anti-pollution properties, in question in this invention, ischaracterised by the fact that the said procedure, in combination withthe production steps usual for the production of traditional ceramics,comprises the following phases:

-   -   application of a variable percentage of TiO₂ to the        manufactures' engobe;    -   application of a variable percentage of TiO₂ to the covering        glaze, the silk-screening pastes and the engobe;    -   application, with the covering layer, of particles of material        designed to increase the refraction of the solar light to which        the manufactures are exposed;    -   addition of substances designed to absorb NOx to the covering        layer and/or to the material of which the engobe is composed;    -   creation of micro channels in the covering layer of the ceramic        manufactures, said micro channels being designed to increase the        permeability to water of the said manufactures;    -   realisation of micro uneven areas in the aforesaid covering        layer, said micro uneven areas being designed to increase the        exchange surface between the single manufacture and the        atmosphere;    -   insufflation of air, on certain ramps of the kiln, during the        traditional firing at 1200° C.; said insufflation being designed        to produce an improvement in the photocatalytic effect of the        TiO₂.

These and other characteristics will better emerge in the descriptionthat follows of a preferred embodiment illustrated, purely in the formof a non-limiting example.

After a first thermal treatment of a traditional type at lowtemperatures, designed, substantially, to facilitate the evaporation ofat least part of the humidity present in the unfired tiles, theprocedure continues with the application of the engobe and a glaze inwhich there is substantially 25% TiO₂, preferably in the form ofAnatase; this application is carried out, preferably, by means oftraditional methods (for example, a disk booth) or by means ofairbrushes without air with suitably modified nozzles. There may beMagalite in the engobe.

There is silica sand mixed in with the glaze and, possibly, also whitepigments. These materials may also be mixed in with the engobe.

The application of the TiO₂ with the covering layer of the tiles, forexample the silk-screening layer, envisages a presence of a percentageof the said TiO₂, limited to the materials constituting the said layer,which may vary, substantially, from 20% to 100%.

In the said glaze application phase, Zeolite and/or Petalite are addedfor the purpose of increasing the effect of the Magalite mixed in withthe engobe.

Contemporaneously, and also in the aforesaid covering layer, microchannels and uneven areas are produced.

Finally, then, during the application of the covering layer, usingsilk-screening machines of a commonly known type, four operations areperformed contemporaneously by means of the use of four synchronisedsilicon rollers, in the following order: a first roller creates themicro uneven areas on each tile base, a second roller applies thesubstance(s) designed to increase the absorption of NOx, a third rollerapplies the material designed to increase the refraction and a fourthroller compacts everything, redefines the micro uneven areas andproduces the micro channels.

At this point, the definitive firing takes place, the said firing beingof the traditional type as regards the temperature, which, for porcelainstoneware ceramic material, reaches around 1,200° C., but in theprocedure in question in this invention, envisages a modificationconsisting in an insufflation of air directly into the firing kiln; saidinsufflation involves the use of a system of shutters positioneddirectly above the kiln and operated by software which controls, at thesame time, the oxidisation, the quantity of CO₂ and the TiO₂ meltingpoint.

As the last phase of the procedure in question in this invention, are-firing of the tiles may be effected at approximately 600° C., subjectto the application, to the tiles fired the first time, of a thin layerof crystalline containing TiO₂.

Over the course of the description, explicit reference has also beenmade to porcelain stoneware ceramic tiles as the ceramic manufactures,but the procedure in question in this invention can quite evidently alsobe applied, advantageously, to ceramic tiles of a different type, forexample single-fired, monoporous, double-fired, clinker tiles etc.

Moreover, over the course of the description, explicit reference hasbeen made to tiles, but it is evident that the procedure in question inthis invention is applicable, advantageously, to any type of ceramicproduct regardless of the form and dimensions.

The applicant has proceeded with the production of three classes ofporcelain stoneware tiles:

-   -   a) a tile without any modifications to the traditional firing        and with the presence of TiO₂ essentially in the form of Rutile;    -   b) a tile obtained with the modification of the firing phase by        means of the insufflation of air, intervening during the        transformation of Anatase into Rutile;    -   c) a tile, as in the previous point, but with the addition of a        layer of TiO₂ after firing is complete. Tests have also been        carried out in which this last typology of tile undergoes a        re-firing at 600° C. with the aim of improving the fixing of the        TiO₂ still further.

The three typologies of tiles just mentioned underwent efficiency testswhich lead to the conclusion that 100 m² of treated tiles, withparticular reference to those in typology c), can clean a volume of airof approximately 15,000 m³ during a sunny day.

There will now follow a list of the production characteristics of thefive embodiments originating from the aforesaid three tile typologies. Afirst porcelain stoneware manufacture was obtained with the followingproduction characteristics:

-   -   engobe with 25% TiO₂ applied by means of an airbrush without        air, functioning under high pressure;    -   serigraphy using iron molybdate;    -   calcic glaze with 25% TiO₂ applied by means of an airbrush        without air, functioning under high pressure;    -   application of 100% TiO₂ by silk-screening.

A second porcelain stoneware manufacture was obtained with the followingproduction characteristics:

-   -   engobe with 25% TiO₂ applied by means of a disk booth;    -   silk-screening using iron molybdate;    -   zinc glaze with 25% TiO₂ applied by means of an airbrush without        air, functioning under high pressure;    -   application of 100% TiO₂ by silk-screening.

A third porcelain stoneware manufacture was obtained with the followingproduction characteristics:

-   -   engobe with 25% TiO₂ applied by means of a disk booth;    -   silk-screening using iron molybdate;    -   glossy alkaline silica-boron glaze with 25% TiO₂ applied by        means of an airbrush without air, functioning under high        pressure;    -   application of 100% TiO₂ by silk-screening.

A fourth porcelain stoneware manufacture was obtained with the followingproduction characteristics:

-   -   engobe with 25% TiO₂ applied by means of an airbrush without        air, functioning under high pressure;    -   silk-screening using iron molybdate;    -   glossy silica-boron-zirconium glaze with 25% TiO₂;    -   application of 100% TiO₂ by silk-screening.

A fifth porcelain stoneware manufacture was obtained with the followingproduction characteristics:

-   -   engobe with 25% TiO₂ applied by means of an airbrush without        air, functioning under high pressure;    -   silk-screening using iron molybdate;    -   application of 100% TiO₂ by means of an airbrush without air,        functioning under high pressure;    -   application of 100% TiO₂ by silk-screening.

The advantage of the procedure in question in this invention consists inthe fact that it enables the realisation of ceramic manufactures forfinishing in the building industry capable of developing aphotocatalytic oxidising action against polluting and bacterial agents.

1. A method for the manufacturing of porcelain stoneware tiles withanti-pollution properties, characterised by the fact that the methodcomprises the following steps: applying a variable percentage of TiO₂ ina covering glaze with a silk-screening paste and engobe finish to a tileto provide a covering layer on the tile; adding particles of material tothe covering glaze designed to increase the refraction of the solarlight to which the tiles are exposed; adding substances designed toabsorb NOx to the covering layer and/or to the material of which theengobe is made; creating micro channels in a thickness of the coveringlayer of the tile, said micro channels being designed to increase thepermeability to water of said tiles; creating micro uneven-areas in thethickness of the aforesaid covering layer, said micro uneven areas beingdesigned to increase the exchange surface between the single tile andthe atmosphere; insufflating or blowing air on the covering layer duringthe traditional firing at 1200° C.; said blowing of air being designedto produce an improvement in the photocatalytic effect of the TiO₂, thepresence of the TiO₂ in the single tile varies from 1% to 25% and isused un the form of anastase, the materials designed to increase therefraction of the solar light to which the tiles are exposed comprisewhite pigment and particles of silica; said pigments and said silicaboth being applied to the same ceramic tile, the application of the TiO₂to the covering layer is obtained by means of a brush without air,functioning under high pressure; and the engobe is applied by means of adisk booth, the application of the materials designed to increase therefraction of solar light to which the tiles are exposed is obtained bymeans of silk-screening machines designed to apply glaze by means ofsilicone rollers according to the thickness wanted, the substances addedto the covering which are designed to facilitate the absorption of theNOx are magalite and one of zeolote or petalite; said magalite is mixedin with the engobe, while the zeolite or petalite are mixed in with theglaze, and the application to the glaze of materials designed toincrease the refraction of solar light and of substances designed toabsorb NOx, and the creation , also in the covering layer, of microchannels and uneven areas are obtained simultaneously through the use offour synchronized silicon rollers in the following order a first rollercreates uneven areas on the base of every single tile, a second rollerapplies the substance designed to absorb NOx, a third roller applies thematerial designed to increase the refraction, and a fourth rollercompacts everything, redefines the micro uneven areas and produces microchannels.
 2. A ceramic tile made according to the method of claim 1.